The present invention is concerned with the communication of quadrature modulated digital data, and more particularly to the acquisition and tracking of two independent digital bitstreams communicated using quadrature phase shift keyed (QPSK) modulation or the like.
Quadrature modulation techniques are well known for the transmission of digital data. In quadrature modulation, two carrier components are modulated ninety degrees apart in phase by separate modulating functions. One of the many applications in which quadrature modulation techniques can be useful is in the communication of digital television signals, such as high definition television (HDTV) signals. The digital transmission of standard television signals, for example under the National Television Systems Committee (NTSC) format, can also benefit from the use of quadrature modulation.
In a communication system employing quadrature modulation (such as QPSK modulation) to transmit two independent digital bitstreams, coding schemes may be employed to provide error detection and correction at the receiver. In order to first acquire and then track a digital bitstream in the presence of noise, a means must be provided to enable the receiver to synchronize to the incoming signal and quickly identify the bitstream of interest. After the receiver "acquires" the bitstream of interest, it is not unusual for a low signal-to-noise level or the presence of an interfering signal to cause the loss of carrier lock in the quadrature demodulator. When this occurs, the receiver must detect the loss of lock and be able to regain the lock as quickly as possible. This is referred to as "tracking" the received signal.
In applications such as the digital transmission of HDTV or NTSC video, reestablishment of synchronization following its loss must occur within a period of milliseconds. Otherwise, significant visible degradation to the video image can result.
It would be advantageous to provide an apparatus and method for rapidly identifying and selecting a desired bitstream from a received quadrature modulated signal. It would be further advantageous to provide such a system that will quickly reidentify and reacquire a desired bitstream following momentary loss of carrier phase lock. It would be still further advantageous for the system to enable the rapid reidentification and reacquisition of the desired bitstream after a slip in carrier phase.
The present invention provides a method and apparatus enjoying the aforementioned advantages.